Have you or she done a trial on any of the estimation calculators?
www.entitledto.co.uk/benefits-calculator/Intro/Home?cid=3b0501e5-5db9-4b7b-bcf4-8e37f847e6d3
benefits-calculator.turn2us.org.uk/AboutYou
BUT remember they are just a guide
so I assume she needs to ask the landlord if he can just put the tenancy in just her name?
Never assume! The landlord can deny her this if she can't pay the rent. Takes time to evict but sadly too many landlords don't do so legally especially a vulnerable young woman. I'd urge caution here, also with regard to her personal safety once landlord knows she's there alone, can she lock up from inside so as even with a key he couldn't get in? Hopefully she has a decent one but literally anyone can be a landlord so she needs to be careful.
On the rent/tenancy side of things shelter are very knowledgeable and helpful.
She needs to ASAP reduce/remove all expenses that are directly related to now ex boyfriend so she's not liable for them for any longer than necessary:
Council tax reduction
Any pay tv subscriptions that were purely for him
Gaming subscriptions
Any debts that are his but coming out of her or a joint account.
If they have a joint account that needs changed ASAP or she runs the risk of him running up debts on it
Any joint credit cards?
She needs to sit and do a new budget being realistic and cutting out luxuries and being frugal on necessities, if she's not already she needs to learn to be a savvy consumer
Also maximising income - picking up extra shifts, loyalty cards for wherever she shops, if she has time/ability doing surveys etc online...
She should be ok for now but I agree a houseshare would be a more sensible option.
Does she have a friend who is looking to move out of parents or is unhappy in current living situation who might be willing to rent a 2 bed with her and the ferrets?
Does the landlord know about the ferrets?
Lots to think about